1. Field of the Invention
The present invention relates to radio frequency communication receivers, systems and methods employing ultra wide band (UWB) signaling techniques. More particularly, the present invention relates to systems, methods and computer program product configured to remove “narrowband” energy from a broader spectrum containing a UWB signal, in a UWB receiver.
2. Description of the Background
Wireless communication systems operate on the principle of using a transmitter that is configured to take data and send the data to an amplifier and antenna, which converts the data from electrical signals into electromagnetic radiation. This electromagnetic radiation propagates through the air and is converted from electromagnetic radiation into an electric current (or voltage) by an antenna coupled to a receiver. The electrical signals from the antenna are typically very small and therefore need to be amplified before being sent to a detector for converting the electrical signals into digital information (or the type of information that comprised the source signal).
Accordingly, the receiver converts all the energy that is passed from the antenna into an electrical form and then identifies where the useful information is contained within the energy coupled from the antenna to produce a useful output representative of the input signal. A problem arises if an unintended signal, particularly a strong signal, is coupled into the antenna at the same time as the desired signal. In this case, the unintended signal, if in the frequency spectrum that coincides with the intended signal, will tend to “jam” the radio, thereby reducing reception quality. Furthermore, even if the unintended signal is not coincident in frequency with the desired signal, the unintended signal may nonetheless drive a low noise amplifier, which is typically coupled to the antenna, into a saturation mode. When this low noise amplifier (LNA) is saturated, the gain of the LNA is reduced and the low noise amplifier creates intermodulation products and harmonics. Both effects degrade the reception of the intended signal. Accordingly, in most communication systems it is desired to avoid driving the radio front-ends into saturation by overdriving an amplifier into a non-linear mode of operation.
In narrowband communication systems, one technique for avoiding the saturation of a front end amplifier due to out-of-band radio frequency interference (RFI), is to equip the radio front end with a bandpass characteristic centered around the intended signal, but excluding the unintended RFI. However, such techniques are not suitable if the intended signal is relatively wideband, because the interfering signal has spectra within the spectra of the desired signal. This phenomena happens in a conventional spread spectrum system, much like a CDMA system or other direct sequence spread spectrum system, or even a frequency hopping system.
Another way that a conventional receiver front end can deal with relatively large “in band” interferers is to provide an automatic gain control (AGC), so that the amount of gain in the receiver front-end is reduced, thus avoiding amplifier saturation. However, the problem arises that the RFI may be so high that the difference in magnitudes between the RFI and the desired signal is beyond the dynamic range of the receiver circuitry. Furthermore, AGC cannot affect the signal to interference ratio (SIR) because it affects the two equally.
Another technique for dealing with in-band RFI in broadband communication systems is to first detect and then suppress the RFI. However, such systems usually require a detector to distinguish an interferer from an intended signal and special cancellation circuitry dedicated to the function of “notching” the unintended RFI. Inserting notch filters into the passband creates not only detrimental insertion loss, therefore increasing the noise for the radio front end, but also introduces phase distortion (and corresponding time sidelobes) into the received signal thus limiting the effectiveness of such systems. Furthermore, lower cost systems are usually not adaptive because it is difficult to adjust the center band of the notch frequencies based on the particular interfering signal at any given time.
Spread spectrum communication systems have a predetermined amount of “processing gain,” which relates to the amount of redundancy in a transmitted signal. In direct sequence spread spectrum communication systems, this amount of redundancy in the signal, materializes in the form of a much broader bandwidth used to communicate the signal than is necessary if simply the information itself were transmitted (in a “narrowband” modulation format). Accordingly, the receiver applies the despreading code to the received signal to “despread” the signal (i.e. correlate to the desired signal) and suppress the RFI. The RFI is suppressed because the RFI does not coherently combine on a chip-by-chip basis with the spreading code (i.e. it is mostly uncorrelated to the desired signal). More detailed descriptions of spreading techniques and systems for employing spread spectrum communications are provided in “Spread Spectrum Design LPE and AJ Systems,” by David L. Nicholson, Computer Science Press, 1987, ISBN0-88175-102-2, the entire contents of which being incorporated herein by reference.
FIG. 1 is a plot of an idealized receiver front end transfer function 10. As can be seen, signals falling in the bandwidth of the receiver front end 10 are coupled into the receiver and processed by receiver. Accordingly, a broadband signal 11, for example, which has frequency components from flo to fhi are coupled into the receiver front end. However, if other signals are also present such as RFI1 and RFI2, then these larger narrowband signals also have to be processed by the receiver. Accordingly, the receiver components need to have sufficient linearity to handle signals that range from the maximum peak of either interference signals RFI1 or RFI2 to a lowest signal level from an intended signal 11. Furthermore, because RFI1 and RFI2 are “in-band interferers” (meaning that these interferers overlap in frequency with the desired signal 11), the detection circuits also will have to attempt to decode data transmitted in intended signal 11 while in the presence of RFI1 and RFI2. It would be possible to include a notch-filter 12 that would “excise” RFI1 from the received band. However, a problem with this approach is that the notch filter 12 destroys part of the desired signal and will increase the noise effect by having a predetermined amount of insertion loss and will “notch” a certain predetermined amount of signal energy in the excised band.
FIG. 2 is a spectral plot of a spectral power density of various RFI in the Alexandria, Va. area. The RFI components are distributed between 0 and 1 GHz. Selected bands within this portion of the spectral band are noted in FIG. 2. As can be seen, there are a number of large narrowband interference sources that would give rise to in-band, or even out-of-band radio front-end saturation problems. Accordingly, in conventional UWB receivers the high-levels of RFI and large number of RFI sources are an accepted part of the UWB communications band. As will be seen, the present inventors refused to accept the RFI-contaminated band as being an unavoidable feature of the UWB band, and have use devised a radio-front end that cancels RFI prior to (as well as in addition to) despreading operations in the receiver.
FIGS. 3(a) and 3(b) show corresponding frequency-domain and time-domain plots of tones used to characterize a radio front-end frequency response of receivers. For example, in FIG. 3(a), the receiver front end has a relatively flat transfer function which is characterized typically by identifying a magnitude for a particular frequency (e.g. flo) when a tone at that frequency is presented to the device being characterized (in this case a receiver front-end). Thus, the transfer function magnitude shown in FIG. 3(a) can be said to have a relatively flat passband that would pass all frequencies between flo and fhi.
FIG. 3(b) shows a typical sinusoidal signal that would be used to perform spectrum analysis when used to characterize the transfer function of the receiver front end in FIG. 3(a). Typically, a network analyzer is used to apply a particular time waveform (such as a sinusoid as shown) to the device being measured for a predetermined number of cycles at a particular frequency. The amplitude and phase at a particular frequency is measured for a predetermined period of time before the continuous tone signal is changed to another frequency. The set of measurements spanning a range of frequencies characterizes the filter transfer function. A point to be made here is that classical characterization of receiver front ends is performed by way of spectral analysis that presumes a certain persistence in the spectral component of a tone being applied to the device being characterized.
Conventional UWB communication systems transmit energy over a much larger bandwidth than a normal “narrowband,” or even a spread-spectrum communication systems' transmission bandwidth. Accordingly, it would be expected that the number of narrowband signals to be encountered by such UWB bands would be relatively high. Examples of such UWB systems include deRosa (U.S. Pat. No. 2,671,896), Robbins (U.S. Pat. No. 3,662,316), Morey (U.S. Pat. No. 3,806,795), Ross et al. (U.S. Pat. No. 5,337,054) and Fullerton et al. (U.S. Pat. No. 5,6777,927).
The challenge, as presently recognized, is to correctly distinguish an intended, transmitted signal at the UWB receiver in the presence of narrowband tones that interfere with the intended signal, while not adversely altering the nature of the intended UWB signal.